The present invention relates to optical microscopy and, more particularly, to photoluminescence microscopy and spectroscopy used to map material quality in semiconductor wafers and devices.
In the information age, communications technology depends on the efficient manufacture of photonic and electronic devices. Optical testing promotes manufacturing efficiency by controlling the quality of incoming materials, providing rapid feedback for process improvements, and analyzing why a product has failed. New, non-destructive optical techniques are being used to measure key properties of semiconductor materials and devices. Optical mapping reveals defective regions in various types of wafers, as well as in optoelectronic devices, such as lasers, modulators and detectors, used in lightwave communications systems.
Spatially resolved photoluminescence is particularly valuable for measuring and characterizing the radiative uniformity of optoelectronic materials, wafers, epitaxial layers and devices. In spatially resolved photoluminescence, the semiconductor wafer or device is irradiated with a beam of highly focused monochromatic light having energy greater than the bandgap energy of the semiconductor. As this so-called xe2x80x9cpumpingxe2x80x9d beam is scanned over an area of the wafer, broadband photoluminescence from the surface is detected, with changes in the intensity of the photoluminescence indicative of defects, such as dislocations. Moreover, displaying the intensity of the photoluminescence on a video monitor creates a spatially resolved photoluminescence image.
In the prior art, confocal laser microscopes have been adapted to detect photoluminescence with high spatial resolution. For example, shown in FIG. 1, is a confocal scanning laser microscope 100 which utilizes its confocal properties to separate the photoluminescence originating from different points on the semiconductor. In this latter configuration, light 105 from laser 110 is focused through pinhole 115 by lens 120, and then into a diffraction-limited spot on the surface of semiconductor 125 by objective lens 130. Photoluminescence from the semiconductor surface, and within this diffraction limited spot, is directed by dichroic beamsplitter 135, and focused by objective lens 130 through pinhole 140 onto detector 145. Dichroic beamsplitter 135 transmits light from laser 110, but only directs longer wavelength photoluminescence onto detector 145.
Photoluminescence substantially only originating from focused spot 150 passes through pinhole 140. Light from any other point on the semiconductor is blocked by the edges of the pinhole inasmuch as focused spot 150 is confocal with pinholes 115 and 140, giving this configuration the ability to obtain highly spatially resolved photoluminescence images.
Spectrally resolved photoluminescence, on the other hand, is generally used to measure the compositional characteristics of the semiconductor material, such as bandgap energy, donor and acceptor energy levels, phonon energy, and the like. In spectrally resolved photoluminescence, highly monochromatic light is used to excite an area of the semiconductor, with the resulting photoluminescence directed into a monochromator which resolves the spectral components of the photoluminescence. For example, certain photoluminescence systems sold by Waterloo Scientific Inc., focus the photoluminescence onto the entrance slit of a grating monochromator.
Unfortunately, such prior art photoluminescence systems do not readily allow an operator to make both spatially as well as spectrally resolved photoluminescence measurements, particularly with spatial resolutions in the micron (xcexcm) region. Additionally, such photoluminescence systems only pass a fraction of the collected photoluminescence to the detector. However, a spectrally resolved photoluminescence mapping of semiconductor wafers with good spatial resolution and collection efficiency has been recently disclosed by Dixon et al. in U.S. Pat. No. 5,192,980, which is incorporated herein by reference. Referring to FIG. 2, Dixon et al. integrates a monochromator or spectrometer into the detection arm of a confocal microscope adapted to obtain highly spatially resolved photoluminescence spectra of the semiconductor. More specifically, a parallel beam 155 of laser light passes through a beamsplitter 160 to enter objective lens 165 which focuses the beam to a diffraction limited focal spot 170 on the surface of a semiconductor specimen 175. On-axis photoluminescence 180 from the specimen is collected by objective lens 165, and is then partially reflected by beamsplitter 160 into a detection arm 185 of the microscope. On-axis photoluminescence 180 strikes diffraction grating 190 which diffracts the light toward lens 195, placed a focal length, f, in front of a pinhole 200. That is, pinhole 200 is confocal with focal spot 170 at the focal point of lens 165.
Inasmuch as diffraction grating 190 separates incoming light 180 into its spectral components along a longitudinal axis, only photoluminescence of a narrow wavelength band passes through pinhole 200, and reaches detector 205. Shown in FIG. 2 are exemplary spectral components 210 (xcex1+xcex94xcex), 215 (xcex1), 220 (xcex1xe2x88x92xcex94xcex), with only spectral component 215 (centered at xcex1) passing through pinhole 200. Light centered at any other wavelength emitted from focal spot 170 hits the area surrounding pinhole 200, and is not detected.
Unfortunately, the photoluminescence system of Dixon et al., and similar confocal based photoluminescence systems, are generally ill-suited for semiconductor specimens exhibiting substantial lateral carrier diffusion.
In accordance with the teachings of the present invention, it has been discovered that when the lateral carrier diffusion area exceeds the diffraction limit spot of the illuminating beam, an optical analysis that includes the extended size of the carrier diffusion area provides a technique for more properly controlling the size of the PL collection region or the axial photoluminescence resolution. On the above basis, the optical system is uniquely characterized in that the optical fiber(s) within the detection arm(s) of the optical system functions as the effective field stop for off-axis photoluminescence. As the field stop, the optical fiber(s) limits the size or field of view of the photoluminescence corresponding to a PL collection region of desired radius. Importantly, it does so, by limiting the cone of light accepted from the off-axis photoluminescence vis-a-via its core size and numerical aperture.
Thus, the underlying rational in controlling the spatial resolution for an extended region of photoluminescence is to judiciously choose the core radius, and the numerical aperture of the optical fiber(s) so as to control the conical bundle of rays from points on the periphery of the desired photoluminescence region so as to reach and enter the optical fiber(s).
In a preferred embodiment, light from a laser is focused into a diffraction limited spot and scanned on the surface of a semiconductor specimen in a predetermined pattern. The photoluminescence from the semiconductor surface is then directed and focused onto the optical fiber, which by judiciously choosing its core diameter and numerical aperture, functions as the field stop to limit the photoluminescence collection region to a desired radius. Photoluminescence spectroscopy and/or microscopy is performed by coupling the collected photoluminescence within the optical fiber into an optical spectrum analyzer and/or photodetector, using optical coupling means or optical switches.
In one embodiment, since collected photoluminescence incident outside the core of the optical fiber is not detected, limiting the PL collection region to a desired radius requires that the photoluminescence from any point outside the desired radius has its conjugate image also outside the core radius. To do so, requires setting the ray height of the principal ray of the collected photoluminescence from the periphery of the PL collection region to the core radius of the optical fiber. This constraint requires, however, that some of the collected photoluminescence falls within the acceptance cone or numerical aperture of the optical fiber.
Alternatively, the numerical aperture of the optical fiber can be tailored to control the radius of the PL collection region, instead of the core radius. At some off-axis point of the PL collection region, no light will be accepted by the optical fiber since all rays incident on the core exceed the maximum external slope of the ray that can be totally internal reflected within the fiber. In this later case, the minimum ray angle of the off-axis marginal rays from the periphery of the desired PL collection region should just match the numerical aperture NAfiber of the optical fiber. The photoluminescence from the periphery of the desired collection region, however, should reach a conjugate image point inside the core.
One advantage of the present invention is that it can readily utilize fiber technology to enhance performance. For example, optical amplifiers can be built into the optical fiber(s) to increase the signal level of the collected photoluminescence, without the need to increase the power level of the light used to excite the semiconductor specimen. Additionally, a spectrometer may be fabricated within the optical fiber, allowing more efficient light path for spectrally resolving the photoluminescence, as well as resulting in a more compact system.